The present invention relates to an olefin polymerization catalyst, its preparation process and application thereof In particular this invention relates to a bis-Schiff base ligand-containing transition metal compound, its preparation process and a process for homopolymerization or copolymerization of olefins with said compound as a major catalyst.
In the field of olefin polymerization, non-metallocene olefin polymerization catalysts exhibit certain advantages in some aspects over metallocene olefin polymerization catalysts, e.g., a much wider range of the ligand for synthesizing the catalyst, capability of some complexes to catalyze the copolymerization of polar monomers with xcex1-olefins, capability to catalyze the oligomerization of ethylene by changing the substitution group of the ligand, and so on. V. C. Gibson (Angew. Chem, Int. Ed. 1999, 38 p428) comprehensively summarizes various catalytic systems for non-metallocene olefin polymerization, wherein the active center metal involves groups IIIB-XIB transition metals. CN1225645A discloses a post-transition metal catalyst for olefin polymerization formed by a bidentate ligand such as an imide and a central metal being selected from Groups IIIB-XIIB metals.
EP0874005A discloses a bis-schiff base [N,O]-coordinated olefin polymerization catalytic system for the first time, wherein the metal element M is selected from IIIB-XIB metals. The disclosed non-bridge complex has the following structural formula; 
wherein R1 is selected from hydrogen, halogen, and alkyl. This patent application especially points out that the substitution group adjacent to the hydroxyl is preferably halogen or C3-C20 branched alkyl if there is no substituent or the substituent which is a smaller alkyl such as methyl, ethyl and the like at this position, the polymerization activity of the system is low. This type of catalyst has major advantages of providing a same catalytic activity compared to that of an ordinary metallocene for the catalytic polymerization of olefins, and resulting in much higher molecular weight of the obtained polyethylene with broader range of distribution of the molecular weight as compared to the polyethylene derived with a metallocene catalyst. The shortcomings of such catalyst exists in that the copolymerization property is poor, and when it is used for the copolymerization of ethylene with xcex1-olefins, the molecular weight decreases to a large extent and the insertion rate of the comonomer reached only less than 4 mol %.
An outstanding merit of the metallocene catalyst system lies in the possession of a very high copolymerization power, which is favorable to obtaining low-density polyethylene plastomer, even elastomer. However, the fatal weakness of the metallocene catalyst is that the molecular weight of the polymer drops so sharply along with the increasing insertion rate of the comonomer when the catalyst catalyzes the copolymerization of ethylene with xcex1-olefins with a result that it is hard to simultaneously obtain polyethylene with both a high molecular weight and a high content of comonomer (high branching degree).
The object of the present invention is to provide a bis-Schiff base ligand-containing olefin polymerization catalyst and its preparation process.
Another object of the present invention is to provide a process for conducting olefin polymerization by using the aforesaid catalyst
It has been found out through research work that the introduction of a cyclopentadienyl ligand into a bis-Schiff base ligand transition metal catalyst can yield a polyolefin product with a high molecular weight and a high content of the comonomer in the copolmerization of xcex1-olefins. If such a catalyst is used for the copolymerization of ethylene with hexene, the molecular weight Mw of the polymerization product can attain to 570,000 when the insertion rate of hexene is 4.34 mol %.
The olefin polymerization catalyst provided by the present invention has the following structural formula (I), 
wherein R1 is selected from the group consisting of hydrogen, C1-C12 alkly, C1-C12 alkoxy, and C6-C12 aryls, R2 and R2xe2x80x2 are respectively selected from hydrogen and C1-C4 alkyl, R3 is selected from the group consisting of hydrogen, C1-C12 alkyl, C1-C12 alkoxy, and C6-C12 aryl, and M is selected from Ti, Zr and Hf.
R1 in formula (I) is a substitution group on the benzene ring of salicylidene; preferably hydrogen or C1-C6 alkyl such as methyl, ethyl, propyl, n-butyl, iso-butyl or tert-butyl. The substitution can be at 3-6 position, preferably 3 or 5 position, i.e. the substitution is at the para- or ortho-position of the hydroxyl.
R2 and R2xe2x80x2 in formula (I) are substitution groups on the aniline ring, preferably hydrogen, methyl ethyl, propyl, iso-propyl, n-butyl, tert-butyl or iso-butyl. R2 and R2xe2x80x2 can be identical or different. If they are identical, it is preferred that both are hydrogen, methyl, ethyl, tert-butyl or iso-propyl. If they are different, it is preferred that one substitution group is hydrogen or a C1-C4 alkyl, and the other is a C1-C4 alkyl, e.g., one is hydrogen and the other is methyl, ethyl, or propyl.
R3 is a substitution group on the frame of cyclopentadiene, preferably hydrogen or a C1-C4 alkyl such as methyl, ethyl, and more preferably hydrogen.
M is a transition metal, preferably Ti, or Zr.
The process for preparing the catalyst of the present invention comprises the following steps:
(1) Reacting a Schiff base ligand compound shown by formula (II) with an alkyl alkali metal compound in an organic medium to form an alkali metal salt of a Schiff base ligand. R1 in formula (II) is selected from the group consisting of hydrogen, C1-C12 alkyl, C1-C12 alkoxy, and C6-C12 aryl, and R2 and R2xe2x80x2 are selected from hydrogen and C1-C4 alkyl respectively 
(2) Reacting the alkali metal salt of the Schiff base ligand with a cyclopentadienyl metal chloride having a formula of CpMCl3 in an organic medium, removing the solvent, washing the residue with an organic solvent, filtering the resultant, and recrystallizing the filtrate. In said CpMCl3, M is Ti, Zr, or Hf, Cp is a mono-substituted cyclopentadienyl, and substitution group R3 is selected from the group consisting of hydrogen, C1-C12 alkyl, C1-C12 alkoxy, and C6-C12 aryl.
Step (1) relates to the reaction for preparing the alkali metal salt of the ligand, wherein the suitable reaction temperature is xe2x88x9278-25xc2x0 C. and in particular, a low temperature should be maintained at the beginning of the reaction to prevent the rapid elevation of the reaction temperature and the formation of byproducts. Among said alkyl alkali metal compounds, alkyl lithium is preferred with butyl lithium being more preferred. In the synthesis of the ligand salt, the molar ratio of the ligand to the alkyl metal compound is 0.1-2.0:1 and the reaction time is preferably 0.1-24 hours.
The suitable temperature in reaction step (2) is xe2x88x9278-25xc2x0 C., and the reaction time is preferably 0.5-24 hours. The molar ratio of the alkali metal salt of the Schiff base ligand to CpMCl3 in the reaction is 1.0-2.0:1. Among the CpMCl3 compounds, Ti is preferred for M and cyclopentadienyl is preferred for Cp.
The organic medium used in aforesaid steps (1) and (2) is selected from tetrahydrofuran and ethyl ether, preferably tetrahydrofuran. The organic solvent used for washing in step (2) is selected from the group consisting of ethyl ether, benzene, toluene, n-hexane, petroleum ether, and the mixture thereof.
The process for preparing the complex of formula (II) in step (1) comprises dehydrating condensation of the corresponding aniline compound of formula (II), such as aniline, 2,6-di-isopropyl aniline with the corresponding salicylal compound in equal mole ratio in ethanol at the reflux temperature for 0.1-10 hours; after the reaction, cooling the resultant to the room temperature, filtrating precipitated solid and removing the solvent to yield the ligand compound of formula (II).
The ligand compound preferred in the present invention includes salicylidene aniline, salicylidene 2,6-di-isopropyl aniline, salicylidene 2-isopropyl aniline, salicylidene 2,6-di-methyl aniline, salicylidene 2,6-di-ethyl aniline, 5-methyl salicylidene aniline, 5-methyl salicylidene, 6-di-isopropyl aniline, 5-ethyl salicylidene 2,6-di-isopropyl aniline, 3-iso-butyl salicylidene aniline, and 3-tert-butyl salicylidene aniline.
Said CpMCl3 compound in step (2) can be prepared by the following process; reacting a sodium salt of cyclopentadiene and its derivative with MCl4 in equal mole in the presence of an organic medium.
The catalyst of the present invention is suitable for the homopoymerization or copolmenization of xcex1-olefins. The polymerization process is to polymerize xcex1-olefins or xcex1-olefins with comonomers by using the catalyst of the present invention as a major catalyst and an alkyl aluminoxane as a cocatalyst under conditions of 10-110xc2x0 C. and 0.1-1.0 MPa. The molar ratio of the aluminum in the cocatalyst to the metal in the major catalyst in polymerization is 100-1500:1, preferably 200-1000:1.
The preferred xcex1-olefin used in polymerization is ethylene or propylene, and the preferred comonomer is butene, hexene, or octene. The polymerization may be conducted by way of bulk polymerization slurry polymerization, or gas phase polymerization.
The present invention will be further described by the following examples without limitation of the scope of the protection of the invention.